Semiconductor contact diode



MarCh 26, 1968 c. o. HULL, JR., ETAL 3,375,47

SEMICONDUCTOR CONTACT DIODE Original Filed Sept. 4, 1964 FIG.|.

a f /1//1 ////f/////// ,//l//f/ 6 2 .nu 0 V/Y f f l f f w 8 O 4. 0 4 64l. w lo B r f /f///////l ///////////////////,/M f 0 f 1 l/f/f l /f/ ll/ f, /f/ 2 Il] RP r \l INVENTORS RR. L Vl m.. E TU M HLW MD ...l RR AEO DF Il NF. E u Mm EC L A VI B United States Patent 3,375,417 PatentedMar. 26, 1968 tice 7 Claims. (Cl. 317-234) ABSTRACT OF THE DISCLOSURE Asemiconductor contact diode comprises a pair of spaced, opposed,metallic electrodes enclosed within a hermetically sealed, glass,cylindrical casing and separated by a silicon semiconductor pelletcontaining both P and N regions so as to form a P-N junction. The Pregion of the pellet abuts an epitaxially formed, monocrystallinecontact button through an aperture in a passivating layer which overliesa lirst major face of the pallet at the P region. The contact button isin turn connected to one of the metallic electrodes through a soldercoating. The N region is connected to the other of the metallicelectrodes through a soldered coating overlying the second major face ofthe semiconductor pellet.

' This application is a continuation of application No. 394,456 filedSept. 4, 1964, now abandoned,

The present invention relates to improvements in semiconductor signaldiodes.

A principal object of the present invention is to provide an improvedsemiconductor junction signal diode which has a very low manufacturingcost and is physically diminutive yet exceptionally resistant tomechanical and thermal shock.

Another object is to pr-ovide a semiconductor diode of the foregoingcharacter which is particularly suited for low cost assembly.

Another object is to provide such a semiconductor diode which includes ahermetically sealed envelope of glass.

Another object is to provide a semiconductor diode of the foregoingcharacter, the parts of which may be assembled by a single heatingoperation which can be conducted in air and which simultaneouslyhermetically seals the glass envelope.

These and other objects of the present invention will be apparent fromthe following description together with the accompanying drawing inwhich:

FIGURE 1 is a fragmentary view, partially broken away in axial section,of a semiconductor signal diode constructed in accordance with thepresent invention;

FIGURE 2 is an axially exploded view, to a diminished scale, of thestructure of FIGURE l;

FIGUREl 3 is an enlarged sectional view of a portion of the structure ofFIGURE 1; and

FIGURE 4 is a view similar to FIGURE 3 showing an alternative embodimentaccording to the present invention.

Referring to FIGURE 1 of the drawing, semiconductor signal diodeconstructed in accordance with our invention includes two identicalsubstantially coaxially arranged oppositely extending electrodes orleads 2, 4 of a metallic composition having a low electric resistivityand having sealing portions 6, 8 capable of being easily hermeticallysealed to glass. A preferred material for leads 2 and 4 is acopper-covered nickel-iron core material known commercially as Dumet Thesealing portions 6, 8 are of equal diameter and form axially facingshoulders 10, 12. To enhance electrical and mechanical contact thereto,the sealing portion 6 includes on its end face at least a partialcovering of a metallic contact layer 14, and the end face of sealingportion 8 is likewise at least partially covered with a similar contactlayer 16. The Contact layers 14, 16 are plated or otherwise adhered tothe end faces of the leads so as to make a good minimum electricalresistance mechanically strong contact with the remaining portions ofthe leads, and the material of contact layers 14, 16 is preferablycopper, silver, or an alloy thereof.

Surrounding the sealing portions 6, 8 of the leads and enclosing thespace between them is a cylindrical casing 40 of glass which ishermetically sealed to the cylindrical surfaces of portions 6, 8 tocomplete the envelope of the diode. The casing 40 can be made of glass,ceramic or other nonporous insulative material, and may preferablyconsist of a glass, having, for example, a working point of less thanabout 1000 C., and a softening point of less than about 750 C, such asCorning 0120 glass or Kimble KG12 glass.

Between the confronting contact layers 14, 16 is situated a wafer-likesemiconductor pellet 18 of semiconductor material such asmonocrystalline silicon or the like. Pellet 18 contains a rectifying P-Njunction 20 between a thin P conductivity type region 22 and an Nconductivity type region 24, the latter of which may include a lowerresistivity N+ portion 25. The periphery of junction 20 is covered atone major face of the pallet 18 by an electrically insulativejunction-protecting and passivating layer 26, such as silicon dioxide,having a central aperture 27 which exposes a portion of P region 22.

Extending upwardly from the surface of pellet 18 exposed throughaperture 27 in layer 26 is an outstanding boss or contact button 50epitaxially formed on P region 22. The epitaxial button 50 is of thesame semiconductor material as that of region 22, and also is of Pconductivity type. The button 50 may be deposited or grown in place onthe surface of pallet 18 exposed through aperture 27 by any suitableprocess, such as, for example, the iodinevapor-transport epitaxialgrowth process described and claimed in co-pending application, Ser. No.385,266, filed July 27, 1964, and assigned to the same assignee as theinstant application. Brieliy, in the process of application Ser. No.385,266, iodine vapor at low pressure of the order of 0.5 to 5millimeters of mercury is used to transport semiconductor material froma source body, having an appropriate conductivity type determiningimpurity concentration, t-o an epitaxial deposition site defined by anaperture in a mask of insulating material such as silicon dioxide, andepitaxial deposition of monocrystalline semiconductor material on thedesired site occurs rapidly and without deleterious effect on the mask.Other suitable epitaxial deposition techniques may be employed to formbutton 50, if desired. The lower portion of the side wall of boss orbutton 50 is fully contiguous with, and conlined by, the peripheral ofside walls 29 of the aperture 27 in layer 26, as best shown in FIGURE 3.The upper portion of the side wall of button 50 may, as shown at 52,extend laterally slightly out over the adjacent top surface ofprotecting and passivating layer 26. Thus the periphery of the interface54 between pellet 18 and button 50 is separated or isolated from theexposed exterior surface of button 50 and layer 26 by at least the depthof the side wall of button 50 in Contact with the side wall 29 ofaperture 27. The layer 26 normally has a thickness of, for example,5,000 to 20,000 angstroms and the button 50 may have a total height suchas` to extend above the top surface of the layer 26 a distance of a fewmils.

On the top of the button is a solder layer 32 which preferably consistspredominantly of a metal Whose eutectic temperature with contact layer14 is less than the sealing temperature of casing 40. Solder layer 32preferably consists predominantly of silver, whose eutectic temperaturewith copper is vabout 780 C. The solder layer 32 is applied to the topof the button 50 by being, for example, electroplated on and alloyed into button 50 in accordance with plating and alloy procedures known tothose skilled in the art. If desired, a thin layer of gold may be platedon button E beneath solden layer 32 to enhance the attachment of thesolder layer 32 to the button. The solder layer 32 may have a thicknessof, for example, 0.1 mil.

The pellet 18 is mounted directly on the end face of one lead 4 by meansof a solder layer 30 which is bonded to the major face of pellet 18remote from layer 26. Solder layer 30 makes a eutectiferous bond withcontact layer 16. The material of solder layer 30 preferably consistspredominantly of a metal, such as silver, whose eutectic temperaturewith contact layer 16 is less than the sealing temperature of casing4t). Solder 30 can contain a small amount, such as 0.1 to 1%, of a donormpurity such as arsenic, if desired, to preclude the formation of arectifying contact between the pellet 18 and the lead 4 to |which it isattached. Solder 30 may desirably also contain a significant portion ofgold, for example, to 40% by weight, preferably provided at least inpart by evaporating or plating of a gold undercoat, shown at 31 inFIGURE 3, onto pellet 18 as a foundation portion of layer 30. The goldundercoat 31 enhances the adherence of layer 30 to the pellet and alsoserves todesrably lower the melting point of the resultingsiliconsilver-gold alloy during sealing of casing 40.

The pellet is mechanically and electrically connected to the end of lead2 by its relatively thick outstanding button 50 of silicon and theeutectiferous bond of solder layer 32 with contact layer 14. Theaggregate thermal coefficient of expansion of the series structureformed by the pellet 18, silicon button 50 and solder layers 30 and 32makes a reasonably good match7 eg., is within a range of from 50 to250%, of the thermal coefficient of expansion of casing 40 within adesired temperature range such as 60 C. to 200 C.

The pellet 18 is preferably so dimensioned that the maximum dimensionacross its major face is slightly smaller than the inside diameter ofthe casing 40, for easy entrance of the pellet into casing 40. Theenlarged diameter sealing portions `6, 8 may each have a diameter of,for example, 32 mils, and a length of 70 mils, and the internal diameterof the casing 40 prior to sealing may be, for example, 34 mils.

The structure above described lends itself Particularly to an assemblysequence which is extremely simple and hence can be accomplished veryeconomically. The lead 4 can be vertically supported on shoulder 12 by asuitable fixture with its sealing portion 8 inserted up into one end ofthe casing 40, and the pellet 18 with the solder layer 30 pre-attachedand the contact button S0 and solder layer 32 pre-attached may be thensimply dropped in the upper open end of the casing 40. Thereafter, thesecond lead 2 may be coaxially inserted into the upper end of the casinginto contact with the solder layer 32. The entire assembly may then be.suitably heated for a brief period, such as seconds at about 850 to 950C. This heating temperature is sufficiently above the eutectictemperature of contact 16 and solder layer 30 to cause the solder layerto alloy and bond to contact layer 16 of lead 4, and likewise to causesolder layer 32 to alloy and bond to the contact layer 14 of lead 2.Simultaneously the heating causes the end portions of the casing to fuseinto hermetic sealing contact with the sealing portions 6, 8 of theleads. The resistance to oxide formation of the silver .in solder 30 andsolder 32 at such temperature particularly facilitates reliable assemblyin this fashion. Adverse effect on the pellet during the heating cycleis avoided by theV short heating time required for complete assembly attemperatures sufficient to seal the Casing and attach the ksolder layer30 and solder layer 32.

During the heat sealing of the mode, a slignr amount of axial pressuremay, if desired, be supplied to compress the pellet 18, solder 30 andsolder 32 between the leads, and facilitate making good solder contactin an air atmosphere. For a button 50 of, for example, a 4-mil diameterat solder layer 32, an axial pressure of about -25 grams is found to bequite sufcient to insure good soldering in an air atmosphere, and theair atmosphere enhances sealing of the glass to lead portions 6, 8.

FIGURE 4 shows another embodiment of the present invention similar inall respects to FIGURE 3 except that the junction between the N regionand P region of the diode pellet is formed at or near the interface 54of button 50 on the remainder of the pellet. In this embodiment, aseparate P region in pellet 18, analogous to P-region 22, is not formedin the pellet prior to deposition of contact button 50. But rather thejunction of the P and N regions is established by the epitaxialdeposition of P- type material of button 50 on the pellet 18, all of thesurface portion of pellet 18 exposed through kaperture 27 being of Nconductivity type prior to deposition of button S0. If desired, anintermediate heat treatment, after deposition of button S0 but beforeenvelope sealing, may be employed to drive some of the acceptor impurityatoms from the P-type button beyond interface 54 and down into theoriginally N-type pellet 18, in which case the junction may have asomewhat shifted location as illustrated by line 56 in FIGURE 4, and theperiphery of the junction may be located beneath layer 26 and slightlyoutside the periphery of aperture 27.

The diode construction above described has many advantages. Use of thesilicon contact button not only eliminates the need for the serpentineresilient connector heretofore frequently required to accommodatethermal ex pansion coefficient differences in diodes having hermeticallysealed envelopes, but it is a particular advantage of the constructionabovedescribed that the silicon Vpellet and the button 50 makes asufficiently good thermal match with the casing' 40 so that the assemblycan be heated up to the casing sealing temperature and cooled downwithout any destructive effects `due to thermal mismatch. Further-more,since the portion of the P-region `adjacent the P-N junction isseparated from the` solder 32 by the substantial thickness of silicon inthe button 50, the possibility is completely precluded of the metal ofsolder 32 alloying into the silicon far enough to affect the P-Njunction, or of otherwise adversely affecting the lo-V cation orelectrical characteristics of the junction during the heat sealing ofthe envelope. Accordingly, the envelope sealing temperature can have ahigher upper limit than is the case with prior art devices having ametal contact closely spaced to the P-N junction and wherein slightalloying of suchk metal contact with the P-region may destroy thejunction or objectionably aifect its electrical properties. In practiceit has been found, for example, that in devices constructed according tothe pres-` ent invention and employing casings of glass, sealingtemperatures as high as 950 C. are feasible, Without incurring anyundesirable effects on the junction properties. This provides aconsiderably greater tolerance on the upper limit of envelope sealingtemperature, and thereby not only simplifies manufacturing equipment andprocedure, but also insures a higher yield of acceptable product. j

The pellet 18 with its contact button 50 and solder portions 30, 32attached does not need toV be oriented end for end before insertion intothe casing 40 because leads Zand 4 are identical, and since the pellet18 is dimensioned to have the maximum dimension of its major facessmaller than the inside diameter of the casing 40, pellet 18 can besimply dropped inside casing 40 and will land on the upfacing end of thelead therein automatically properly arranged and oriented for permanentattachment onto such end face. The pellet 18 doesnot require supportfrom or contact with casing 40 but is attached to and supportedexclusively by the confronting end faces of the leads.

Another advantage of the structure shown is that the direct connectionof the pellet to the leads by the solder region 30 and by contact 50 andsolder 32, and the relatively large transverse dimensions of the solderregion 30, insures a good thermal conductivity path from pellet 18 tothe lea-ds and thus makes it possible for the leads themselves to serveas excellent heat sinks for any heat generated in the pellet duringelectrical operation of the diode, The relatively thick contact button50 also provides a sufficient axial spacing between the pellet and theconfronting end of the lead 2 to keep the lead from touching the pelletat its edge if the lead end face happens to be other than exactly normalto the lead axis. Finally, the reduced interior volume of the diodeconstruction herein described gives it an inherently better resistanceto crushing forces and hence makes it particularly suitable for eventualpotting in an encapsulant with other circuit elements.

It will be appreciated by those skilled in the art that the inventionmay be carried out in various ways and may take various forms andembodiments other than the illustrative embodiments heretoforedescribed. Accordingly, it is to be understood that the scope of theinvention is not limited by the details of the foregoing description7but will be defined in the following claims.

We claim:

1. A semiconductor device comprising a pair of spaced opposedelectrodes, a cylindrical glass casing enclosing the end portions ofsaid electrodes and fused in hermetic sealing contact with saidelectrodes to form an envelope therewith enclosing the space betweensaid opposed electrodes, a body of monocrystalline silicon semiconductormaterial disposed within said envelope between said opposed electrodesand spaced from said glass casing, said semiconductor body having onemajor face and a substantially parallel opposite major face, a layer ofinsulating material on said one major face of said semiconductor bodyhaving a thickness of about 5,000 to 20,000 angstroms and provided wit-ha central aperture, a monocrystalline silicon contact button epitaxiallyformed on the portion of said one major face exposed by said aperture,said contact button extending through said aperture and protrudingbeyond said layer of insulating material in a direction normal to saidlayer a distance of a few mils, said protruding portion laterallyslightly overlapping said insulating layer so as to leave the majorportion of said insulating layer uncovered, a first metallicelectrically conductive contact intermetallically bonded to said one ofsaid electrodes and to said epitaxial contact button, a second metallicelectrically conductive contact intermetallically bonded to saidopposite major face of said semiconductor body and to the other of saidelectrodes, and a P-N junction in said semiconductor material betweensaid epitaxial contact button and said opposite major face of said body,the periphery of said junction terminating at and being covered by saidinsulating layer, the thickness of said button in a direction normal tosaid layer providing a separation of said first metallic electricallyconductive contact from said junction and the protruding portion of saidbutton providing a spacing of said layer from said one electrode.

2. A device as defined in claim 1 wherein said layer is silicon dioxide.

3. A device as defined in claim 1 wherein said electrically conductivecontact bonded to said button includes silver.

4. A -device as defined in claim 1 wherein said epitaxial contact buttonprovides a source of impurity atoms for converting the conductivity typeof the adjacent portion of said semiconductor body, and said P-Njunction lies between said opposite major face and the interface of saidbody and said button.

5. A device as defined in claim 1 wherein said electrodes arecopper-covered, said casing is glass having a Working point of less than1,000 C. and a sealing temperature to said electrodes of 850 C. to 950C., and the intermetallic bonding temperature of said contacts is lessthan said glass sealing temperature.

6. A device as defined in claim 1 wherein said intermetallic bonds havea melting temperature less than the sealing temperature of said casingto said electrode.

7. A device as defined in claim 1 wherein said electrically conductivecontact to said button includes silver and gold, and said intermetallicbonds have a melting temperature less than the sealing temperature ofsaid casing to said electrodes.

References Cited UNITED STATES PATENTS 3,189,973 6/1965 Edwards et al.317-235 3,200,310 8/1965 Carman 317-234 3,265,542 8/1966 Hirshon 317-2343,296,040 1/1967 Wigton 317-234 3,300,841 1/1967 Fisher et al. 317-234JOHN W. HUCKERT, Primary Examiner. I. D. CRAIG, Assistant Examiner.

